Add new pre-trained models for metagenome and transcriptome modes

[lcoombe]

• Metagenome: 2 ZymoBIOMICS mock community datasets (even and log abundances)
  • Same datasets used for previous pre-trained models, and shown in the meta-NanoSim paper
• Transcriptome: Human NA12878 rel2 cDNA data
  • Due to the large size of the dataset, this model is hosted on Zenodo